My default running style is crap. I've never been a great runner, but I'd like to improve my running style and speed.

If I really concentrate I can bound along like an asthmatic gazelle, taking long slow ground covering strides, but as soon as I've run half a mile or so I start to get tired and revert to the default setting which is lots of short steps.

It's not like my run times are totally dreadful as I always pass my PFT well inside the allotted time (mind you the Danish PFT, even the new improved 'harder' PFT is embarrassingly easy), but I'd like to be able to do the bounding along thing for longer than a couple of minutes.

There are two main aspects involved in having a long, ground-eating stride, TT. The first is being tall and/or having long legs in relation to the length of your torso. The second is having relatively short, high calf muscles and thus a long, and very springy, Achilles tendon, as often seen in African long-distance runners. Both of these will result in this long stride you're aiming for.

The bad news is; you're either born with them or not. Your natural stride-length is nigh on impossible to change, because you're basically asking your body, or legs to do work it/they were never originally configured for. What you can change is your stride frequency, but you obviously know that already.

A small tip: Try running with your hips slightly lower to the ground in a sort of semi-Groucho-Marx style. This concentrates the energy of your stride into moving you forward, as opposed to expending some in lifting you up. If you see what I mean.